This invention relates to a novel corona discharge device for depositing charge on an adjacent surface in a controlled pattern and has particular utility in the art of xerography.
Corona discharge devices play an important role in the practice of xerography by establishing or altering the electrostatic charge on adjacent surfaces at several different times during the xerographic cycle. For example, corona devices are generally employed to lay down an initial uniform charge on a photoreceptor, which charge is dissipated in part by exposure to an optical image to form an electrostatic latent image.
After development of the latent image with toner particles, corona devices may also be utilized to alter the charge on the deposited toner prior to transfer of these particles to a copy sheet in order to enhance image transfer and limit background transfer.
Corona devices may further be used to deposit charge on the surface of a copy sheet in order to cause transfer and adherence of toner thereto. In addition, corona devices aid in the stripping or detacking of copy sheets from a photoconductive drum and in neutralizing electrostatic charges on the photoconductor surface prior to the initiation of a new copy cycle.
Conventional forms of corona generating apparatus are illustrated in U.S. Pat. Nos. 2,836,725 and 2,879,395 and generally comprise one or more wire-like electrodes, known as coronodes, horizontally disposed above the surface to be charged and a shield which may take a plurality of different structural forms, partially disposed about the coronode. In one conventional mode of operation, a high voltage D.C. power supply is connected to the coronode with the requisite polarity for the charging operation which is desired, while a conductive layer associated with the surface to be charged is grounded as are the other terminals of the power supply in the shield.
In an alternate mode of conventional corona generating apparatus operation, a D.C. power supply is connected to the coronode and a conductive layer associated with the surface to be charged while the shield is left unconnected or electrically open.
Another arrangement for biasing a corona discharge device in a xerographic machine is taught in U.S. Pat. No. 2,868,989, wherein the corona wire is biased to a high positive corona generating potential, the conductive substrate on which the photoconductive material is held is maintained at ground potential, and the shield is biased to a potential intermediate the corona wire potential and ground.
It should be noted that in each of the above biasing arrangements, the shield is formed of a single integral conducting member which is biased to a constant potential or left unbiased.
Such arrangements have the disadvantage of being static or constant in the charge distributed thereby to an adjacent surface. In some instances, however, it is desirable to charge the distribution of charge delivered to the surface to be charged to create specific charge distribution patterns and to be able to alter these patterns in accordance with the demands of the particular xerographic processor in which the corona devices are used.
Several prior art arrangements for obtaining non-uniform charge distribution are disclosed in U.S. Pat. No. 3,160,746.